The present invention relates to a method and a reactor for mixing one or more chemicals into a process liquid flow. Preferably the method and reactor according to the invention are suitable for introducing and mixing into a process liquid flow one or more chemicals that either alone or together with the material entrained in the one or more process flow tend to form either organic or inorganic layers, accumulations or precipitations onto the wall of the flow tube. The invention is especially preferably suitable for use in e.g. wood processing industry when one or more chemicals of paper and pulp production are being introduced into the production process.
In for example papermaking, as in numerous other fields of industry, there are needs to introduce and in-line mix at least one such substance, hereinafter called a chemical in the widest meaning of the word, into a pipe flow either on its own or that can chemically, electro-chemically or mechanically among themselves or with the materials in the pipe flow form accumulations of solids, precipitations or the like to the wall of the flow pipe or channel. Thus it is to be understood that in the context of this invention the word chemical covers gaseous, substances, liquid substances as well as solid materials. In other words said chemical can, for example, be a substance chemically reacting with a chemical introduced previously or simultaneously into the process flow or one present there initially, or e.g. a retention or adhesive material that is attached to other solid material or particle present in the flow or being introduced into it with the chemical or near it. Thus the idea is also that the word “reaction” covers chemical reactions, electrochemical reactions as well as attachment of particles to each other by means of retention materials and adhesive materials. In other words, all such situations in which the material entrained with the process flow and/or introduced thereto forms material tending to fasten to the surface of the process pipe or structures located therein, is for the sake of clarity called a reaction. In this application the word in-line mixing means mixing made directly in the process pipe taking place, e.g. in papermaking when a chemical is added directly into the pulp flowing in a process pipe towards the headbox of a paper machine. In-line mixing also covers the applications in which the mixing takes place into a liquid or suspension flow of the main or partial process without the product produced in said partial process being stored in immediate storage either alone or together with another product.
In some cases discussed on prior art it is enough to let the desired amount of chemical to flow into a tube flow so that it is mixed with the flowing material, a liquid or a gas, by the turbulence in the actual tube flow. Sometimes the desired amount of the chemical is drained into such a point of a pipe flow where there is a turbulence-producing mechanical apparatus slightly after the chemical addition point, either a static flow hindrance, a rotary mixer or, for example, a centrifugal pump. In some cases the chemical is introduced into a relatively large tank arranged in the process, either directly or, for example, with a substance directed into the tank, whereby the necessary mixer is arranged in the tank.
In all these contexts it is possible that either organic or inorganic precipitations affecting the process accumulate on the wall of either the flow pipe or a special reactor or on some other fixed structure, such as e.g. the chemical introduction means or various structures of the mixer. Such disadvantages can be seen in e.g. that the organic matter starts to decay and spreads microbes into the flow, causing spoilage of the whole end product in the worst case, or in that when released as larger particles the layer spoils the end product by causing e.g. holes in the produced paper (when using paper industry as an example) or disadvantageous changes in the flows of the headbox, causing deterioration of the quality of the end product. The precipitations can also block some process apparatuses or their pipelines fully or partly, increase pumping costs, reduce the effect of chemicals, decrease heat recovery and so on. Further, a layer of a certain kind can form a base for another precipitation, such as a calcium carbonate layer greatly enhances the precipitation of sodium carbonate in black liquor evaporators.
As said problem relating to various precipitations has been known for as long as there have been industrial processes, there have also been attempts to solve it in a number of ways.
One method is naturally to avoid use of materials forming precipitations and to replace them with non-precipitating materials, which however can only ease a marginal amount of the problem cases. In some cases it is not possible to avoid use of precipitating materials and in some cases the replacement materials are so expensive that their use is economically impossible.
Another alternative is to use high-quality raw materials starting from the debarking mill of the pulp mill. In other words, the precipitation problem can be reduced by minimizing the access of the bark layer of the wood into further process. It is also possible to reduce the return of chemicals causing precipitations to use by considering the chemicals causing precipitation problems in chemical circulation. In other words, by considering the precipitation problems in each partial process of the mill, which when using cellulose, pulp and paper industry as example include digester processes, washing and bleaching of brown pulp in addition to chemical recovery, it is possible to considerably reduce the risk of precipitations.
The third alternative is to arrange such flow conditions that precipitations are not allowed to accumulate in e.g. the flow pipe system. In other words, the aim is to plan the flow pipe system so that there are as few places as possible where the flow is so quiet that the solids entrained with the flow can settle against the surface of the pipe system. This method, also, is only limited, because in practice it is impossible to plan the flow pipe system so that no precipitations can be formed. However, good planning allows lengthening washing intervals and the plant downtime required by them.
The fourth alternative is to consider the cleaning requirements of the pipe system already in the planning stage, which allows shortening the required downtime needed for e.g. acid cleaning or the like of the process apparatuses or even manual removal of the layers/precipitations.
The fifth way is to manufacture or coat e.g. the flow pipe system or the reactor with such material that the materials with precipitation tendencies do not attach to it for some reason. The disadvantage of this is, however, that replacing the traditional steel flow pipe or reactor with nearly any other material will multiply the costs.
The sixth alternative is to use such additional chemicals, such as some retention chemicals, chelates, surfactants or inhibitors that one way or the other prevent the formation of precipitations. Such chemicals, however, naturally cause at least additional costs, as they must be continuously measured into the process liquid. Such chemicals can also cause problems in the processing of the process effluents, reusability of the water or in the actual end product, its processing or recycling.
None of the above-mentioned methods has not, however, been found to be totally functional in paper industry, but instead the both flow pipes and process apparatuses as well as reactors and tanks are soiled as precipitations accumulate on their walls.
The above-mentioned precipitation problems have been emphasized when the injection mixers described in, among others, patent publications EP-B1-1064427, EP-B1-1219344, FI-B-111868, FI-B-115148 and FI-B-116473 have been commissioned more widely. The reason for the increase of problems is that while these injection mixers are capable of mixing the chemicals very quickly and evenly into the process flow, the mutual reaction of the chemicals or the reaction with the solids or chemicals in the flow is very fast. Thus there also is a large amount of chemicals simultaneously in the vicinity of the wall of the flow pipe so that as the chemicals form a solids crystal or particle it is attached to the wall of the flow pipe instead of another solid material, such as a fiber or a filler particle. Corresponding chemicals were previously introduced by means of weaker mixers, whereby it took the chemicals tens of seconds, sometimes even minutes, to react mutually or with the solid material or chemical of the flow previously introduced thereto, whereby also the precipitations happening onto the inner surface of the flow pipe were distributed on an essentially longer length of the flow pipe. While the precipitations were previously distributed along nearly the whole length of the process pipes subsequent to the introduction point, they now cover in many cases the surface of the flow pipe for a distance of a few meters from the introduction of chemicals. Because it can be supposed that in the case of both traditional mixing and mixing using injection mixer essentially the same amount of the reaction products of the chemicals is precipitated on the surface of the flow pipe, it is probable that the precipitation layer formed when using injection mixers can in the same period of time be considerably thicker than in the traditional mixing method. Simultaneously the risk of the precipitations being broken up and released as fragments to the flow increases and the occurrence rate of problems caused by the fragments can even increase. This kind of precipitation problems naturally exist in both wood processing industry as well as many other fields of process industry. In fact, nearly all fields of industry where chemicals introduced or mixed into a pipe flow are allowed to react either mutually or with material entrained with the medium flowing in the pipe or a previously introduced chemical, suffer from precipitation problems of the kind described above that in some way affect the propagation of the process or the quality or production of the intermediate or end products.
Precipitation problems of the described kind occur in, e.g. industrial environments where large amounts of various chemicals, including retention chemicals, adhesives and stabilizing agents etc. are introduced and mixed into process liquids.
In addition to papermaking, in wood processing industry such processes include the production, washing, bleaching and chemical recovery processes of mechanical, chemimechanical and microfiber and nanofiber pulp, in which a large amount of precipitation-forming chemical compounds are formed. These can roughly be divided into inorganic and organic compounds.
Of the inorganic precipitation-causing compounds the following can be mentioned in addition to the calcium carbonate already mentioned. Calcium oxalate, the ion of which is formed in oxygen bleaching of chemical pulp. Barium sulphate is formed when bivalent barium cation is removed from the wood material especially when producing pulp in acid conditions prior to bleaching chemical pulp. Presence of alum, i.e. aluminum sulphate increases precipitation tendencies. Precipitating aluminum hydroxide is sometimes formed immediately after the introduction of alum. Alum itself is commonly used in papermaking, such as in some retention programs, as resin glue, coagulant and so on. When these compounds or said compounds, among others, and compounds and chemicals already present in the fibrous suspension or to be later introduced thereto react, the precipitation of the formed new chemicals is usually prevented by using various chemicals. Other inorganic compounds with precipitation tendencies include, among others, calcium sulphate, calcium silicate, aluminum silicate, aluminum phosphate and magnesium silicates.
Organic precipitations most often relate to various particles fastening to the process pipes or other apparatuses and structures in connection with the process flow by means of a bonding agent. The following can be mentioned among such bonding agents: Pitch, being a wood-based material. Most usually pitches are fatty acids or resin acids, even though some other compounds are also traditionally called pitch. Tacky materials are glue or adhesive matter originating in recycled paper. Slimes are materials formed by microbial activity and that are well adapted to the relatively stable high temperatures and relatively constant pH of paper industry. Fungus can be mentioned as the last bonding material. All of the above-mentioned bonding agents causing precipitations are, in a way, materials that have been introduced into the process from the initial materials of the paper to be produced, i.e. they are not intentionally added to the process. In addition to these, there are the chemicals added to the process for other reasons, such as anti-foaming agents, polysiloxanes, mineral oils, retention polymers and chemicals, nanoparticles, microparticles, pH-adjusting agents, brighteners (OBA, FWA), dispersing agents, starches, colouring agents, pigments, waxes, fillers (for example TiO2, CaCO3, talcum, kaoline), minerals and glues (traditional hydrophobic glues: AKD, ASA, resin and synthetic glues) etc. that tend to form precipitations in advantageous conditions either on their own or especially together with the solids or chemicals present in the process.